1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus having a cooling efficiency switching control function.
2. Description of the Related Art
Generally, when an original image is to be copied by use of an image forming apparatus such as an electronic copying apparatus, the image of the original is first exposed, after being placed on an original table, and is scanned by a light source such as an exposure lamp. Then, an image is formed on a photoconductive drum serving as an image carrier, and is developed by a developing unit utilizing a powder-form developer. Simultaneous with this operation (from exposure scanning to development), a copy sheet is fed from a feeding cassette mounted on the outside of the apparatus main body, the developed image being transferred to this copy sheet by a transfer unit or the like, and a predetermined heat and pressure being applied to the sheet in order to thermally fix the developer formed thereon. Thus, the copying process is completed, with the sheet bearing the image being discharged to an externally mounted receiving tray, and any developer remaining on the photoconductive drum being removed therefrom by a cleaning unit, a discharger, or the like.
When the copying process as described above is performed by an electronic copying apparatus, heat is generated by the exposure lamp, the fixing unit, and so on, leading to an increase in the temperature within the main body of the apparatus. In order to dissipate this heat to the outside of the main body, the apparatus is provided with an internal cooling fan. Depending on the termperature within the apparatus main body, the cooling fan is generally turned on/off or switched between high and low speeds so as to maintain the temperature therein within a predetermined temperature range.
A cooling fan which can be switched between high and low speeds can therefore be, at a given time, in one of three different states; namely, at a stop, rotating at low speed, or rotating at high speed, depending upon whether the temperature within the apparatus is at room temperature, at a "standby" temperature, i.e., the temperature at which copying becomes possible, or at a "saturated" temperature, respectively. The "saturated" temperature is defined as a predetermined temperature which is reached--but not exceeded--within the copying apparatus.
First, when the power source of the electronic copying apparatus is turned on, the temperature of the fixing unit therein rises from room temperature to a predetermined level, i.e. the standby temperature, as does also the copying apparatus itself. When the entire apparatus has warmed up to the standby temperature, copying then becomes possible and, in order to ensure that the original table, the surface of the photoconductive drum, and the like do not become excessively heated, the cooling fan begins low-speed rotation.
Subsequently, when copying is started, further heat is generated by the exposure lamp, the fixing unit, and the like, and is dissipated within the apparatus, consequently raising the temperature therein to the saturated temperature level. In response to this increase in heat level, the cooling fan is switched from low-speed to high-speed rotation. When a copying process is completed, the cooling fan is immediately switched from high-speed to low-speed rotation. However, although the heat generated in the apparatus is decreased, the heat accumulated in the apparatus is large, and the temperature in the apparatus is not immediately decreased but is decreased slowly to reach the standby temperature.
In the electronic copying apparatus having a cooling fan which is controlled in the above manner, the fan is rotated at a high speed only during the copying process, resulting in the following problems.
More specifically, when the first copying operation is completed and then the second copying is to be performed shortly after the first copying operation, the second copying operation is performed before the temperature in the apparatus is sufficiently decreased (before the temperature in the apparatus is decreased to the standby temperature). For example, assuming that the first copying operation is completed and the second copying operation is to be performed before the temperature in the apparatus is decreased from the saturated temperature to the standby temperature, the interior of the apparatus is kept at a high temperature for a long period of time since the temperature in the apparatus has not reached the standby temperature before the second copying operation. This is conspicuous when copying is intermittently performed within a short period of time, resulting in a long high-temperature state of the interior of the apparatus. This degrades the charging characteristics of the photoconductive drum and a stable image quality cannot be obtained.